The office environment has, for many years, been the home of objectionable noise generators, viz. typewriters and high speed impact printers. Where several such devices are placed together in a single room, the cumulative noise pollution may even be hazardous to the health and well being of its occupants. The situation is well recognized and has been addressed in the technical community as well as in governmental bodies. Attempts have been made to reduce the noise by several methods: enclosing impact printers in sound attenuating covers; designing impact printers in which the impact noise is reduced; and designing quieter printers based on non-impact technologies such as ink jet and thermal transfer. Also, legislative and regulatory bodies have set standards for maximum acceptable noise levels in office environments.
Typically, impact printers generate an average noise in the range of 70 to just over 80 dBA, which is deemed to be intrusive. When reduced to the 60-70 dBA range, the noise is construed to be objectionable. Further reduction of the impact noise level to the 50-60 dBA range would improve the designation to annoying. Clearly, it would be desirable to reduce the impact noise to a dBA value in the low to mid-40's. The "A" scale, by which the sound values have been identified, represents humanly perceived levels of loudness as opposed to absolute values of sound intensity and will be discussed in more detail below. When considering sound energy represented in dB (or dBA) units, it should be borne in mind that the scale is logarithmic and that a 10 dB difference means a factor of 10, a 20 dB difference means a factor of 100, 30 dB a factor of 1000 and so on. We are looking for a very aggressive dropoff in printer impact noise.
The printing noise referenced above is of an impulse character and is primarily produced as the hammer impacts and drives the type character pad against the ribbon, the print sheet and the platen with sufficient force to release the ink from the ribbon. The discussion herein will be directed solely to the impact noise that masks other noises in the system. Once such impact noise has been substantially reduced, the other noises will no longer be extraneous. Thus, the design of a truly quiet printer requires the designer to address reducing all other noise sources, such as those arising from carriage motion, character selection, ribbon lift and advance, as well as from miscellaneous clutches, solenoids, motors and switches.
Since it is the impact noise which is modified in the present invention, it is necessary to understand the origin of the impact noise in conventional ballistic hammer impact printers. In such typical daisywheel printers, a hammer mass of about 2.5 grams is driven ballistically by a solenoid-actuated clapper; the hammer hits the rear surface of the character pad and impacts it against the ribbon/paper/platen combination, from which it rebounds to its home position where it must be stopped, usually by another impact. This series of impacts is the main source of the objectionable noise.
Looking solely at the platen deformation impact, i.e. the hammer against the ribbon/paper/platen combination, the total dwell time is typically in the vicinity of 100 microseconds. Yet, at a printing speed of 30 characters per second, the mean time available between character impacts is about 30 milliseconds. Clearly, there is ample opportunity to significantly stretch the impact dwell time to a substantially larger fraction of the printing cycle than is typical of conventional printers. For instance, if the dwell time were stretched from 100 microseconds to 6 to 10 milliseconds, this would represent a sixty- to one hundred-fold increase, or stretch, in pulse width relative to the conventional. By extending the deforming of the platen over a longer period of time, an attendant reduction in noise output can be achieved, as will become apparent in the following discussion.
The general concept--reduction in impulse noise by stretching the deformation pulse--has been recognized for many decades. As long ago as 1918, in U.S. Pat. No. 1,261,751 (Anderson) it was recognized that quiet operation of the printing function in a typewriter may be achieved by increasing the "time actually used in making the impression". Anderson uses a weight or "momentum accumulator" to thrust each type carrier against a platen. Initially, the force applying key lever is struct to set a linkage in motion for moving the type carriers. Then the key lever is arrested in its downward motion by a stop, so that it is decoupled from the type carrier and exercises no control thereafter. An improvement over the Anderson actuating linkage is taught in Going, U.S. Pat. No. 1,561,450. A typewriter operating upon the principles described in these patents was commercially available.
Pressing or squeezing mechanisms are also shown and described in U.S. Pat. No. 3,918,568 (Shimodaira) and U.S. Pat. No. 4,147,438 (Sandrone et al) wherein rotating eccentric drives urge pushing members against the character/ribbon/sheet/platen combination in a predetermined cyclical manner. It should be apparent that an invariable, "kinematic" relationship (i.e. fixed interobject spacings) between the moving parts renders critical importance to the platen location and tolerances thereon. That is, if the throat distance between the pushing member and the platen is too great, the ribbon and the sheet will not be pressed with sufficient force (if at all) for acceptable print quality and, conversely, if the throat distance is too close, the pushing member will cause the character pad to emboss the image receptor sheet. Sandrone et al teaches that the kinematic relationship may be duplicated by using a solenoid actuator, rather than a fixed eccentric (note alternative embodiment of FIGS. 14 through 17). Pressing action may also be accomplished by simultaneously moving the platen and the pushing member, as taught in U.S. Pat. No. 4,203,675 (Osmera et al). In addition, Sandrone et al states that quiet operation relies upon moving a small mass and that noisy operation is generated by large masses. This theory is certainly in contravention to that applied in Anderson and Going (supra) and in U.S. Pat. No. 1,110,346 (Reisser) in which a mass multipler, in the form of a flywheel and linkage arrangement, is set in motion by the key levers to increase the effective mass of the striking rod which impacts a selected character pad.
A commercially acceptable printer must have a number of attributes not found in the prior art. First, it must be reasonably priced; therefore tolerance control and the number of parts must be minimized. Second, it must have print quality comparable to, or better, than that conventionally available. Third, it must have the same or similar speed capability as conventional printers. The first and the last factors rule out a printer design based upon squeeze action since tolerances are critical therein and too much time is required to achieve satisfactory print quality.
It is the primary object of the present invention to provide a novel impact mechanism for a quiet impact printer that is orders of magnitude quieter than that typical in today's marketplace, and which nevertheless achieves the rapid action and modest cost required for office usage.